1. Field of the Invention
This invention relates to chelating agents particularly useful for supplying iron and other essential metals to plants growing in calcareous soils.
2. Description of Prior Art
Many soils encountered in various parts of the world are naturally deficient with respect to the availability of certain trace elements essential to growing plants. This deficiency may result from either an actual depletion of those elements in the soil or from a fixation of the trace elements as insoluble salts or organic complexes which cannot be utilized by the plants. Trace element deficiency, particularly iron deficiency, is commonly associated with alkaline calcareous soils which induce the formation of iron oxide and insoluble iron compounds which are unavailable to plant metabolism. The deficiency of iron results in a condition commonly identified as lime-induced iron chlorosis, a condition wherein the area of the leaf between the veins is a marked yellow green in contrast to the dark green of the veins. Iron chlorosis is often accompanied by a deficiency of other metals such as manganese, zinc and copper which are also essential for healthy plant growth. A discussion of possible causes of lime-induced chlorosis is found in "A Decade of Synthetic Chelating Agents in Inorganic Plant Nutrition," Arthur Wallace, Professor of Plant Nutrition, U.C.L.A. (1962) pages 28-35.
One conventional method of correcting iron deficiency has been simply to apply soluble iron salts such as ferrous sulfate to the soil. Such salts are, however, quickly hydrolyzed in alkaline soils to the oxide or hydroxide forms which are unavailable to the plant.
More recently, more effective results have been obtained by utilizing metal chelating agents to solubilize and maintain metals in a form which can be utilized by the plant. Where the soil actually contains adequate amounts of iron but in an insoluble form, the free chelating agent may be added directly to the soil in order to solubilize the iron and make it available to the plant. Where the soil is actually deficient in iron or other essential metals, metal complexes of the chelating agent may be added to the soil and/or directly onto the foliage of the plant.
Many chelating agents have been suggested to correct metal deficiencies in plants growing in calcareous soils. For example
U.S. Pat. No. 3,091,522 suggests carboxylated amines containing a terminal hydroxyl alkyl-sulfoalkyl group.
U.S. Pat. Nos. 2,921,846, 3,005,848, 3,028,407 and 3,248,207 suggest the use of ethylene bis(alpha-imino-o-hydroxyphenylacetic acid) and monoamides thereof.
U.S. Pat. No. 3,472,002 suggests the use of N,N-bis(disubstituted benzyl)-.alpha.-amino acids.
U.S. Pat. Nos. 2,917,528 and 2,964,549 suggest alkanol amino alkane phosphonic acids.
U.S. Pat. No. 3,008,816 suggests alkylene bis(iminosalicylidene) diphosphonic acid.
U.S. Pat. No. 3,038,793 suggests N,N'-bis(2-hydroxy-5-alkylbenzyl) ethylene diamino diacetic acid.
U.S. Pat. Nos. 3,051,563 and 3,403,992 suggest various carboxylated and/or sulfonated amines such as ethylenediamine tetraacetic acid.
U.S. Pat. No. 2,891,854 suggests the use of diethylene triamine penta(acetic acid).
U.S. Pat. No. 3,463,799 suggests dimethylamino-bis(2-hydroxyphenyl) sulfonic and carboxylic acids.
U.S. Pat. No. 3,131,048 suggests various alpha hydroxy carboxylic acids such as glycolic acid.
British Pat. No. 882,987 suggests various free substituted imino dicarboxylic acids.
The above patents are believed to be representative of the prior art relating to agricultural applications of chelating agents designed to correct metal deficiencies in plants, but those skilled in the art will undoubtedly be aware of yet other patents and references not cited herein. Accordingly, these references are cited for purposes of illustration only and are not to be considered a complete or exclusive list of pertinent references. For example, yet another compound not included in the above patents and which has been suggested as an iron carrier is trans-1,2-diaminocyclohexane tetraacetic acid monohydrate. The iron complex of this compound is reputed to be relatively unaffected by calcareous soils and to not form iron hydroxides.
The chelating agents of the prior art, while for the most part having some beneficial effect when applied to the soil of chlorotic plants, are not completely satisfactory in performance. For example, the ethylene diamine tetraacetic acid chelates of tri- and tetravalent metal ions are unstable in neutral and alkaline solutions and hydrolyze to form insoluble hydroxides in calcareous soils. Other compounds are also either unstable under alkaline conditions or are too expensive to be commercially feasible. Of all the chelating agents of the prior art, the ferric chelate of ethylenediamine di(o-hydroxyphenyl acetic acid) (hereinafter EDDHA) is one of the most effective in overcoming iron deficiency in calcareous soils, and this compound is generally used as a standard in the evaluation of new chelating agents. The use of EDDHA on calcareous soils, however, has the disadvantage of tending to induce low manganese levels in treated plants unless the application of EDDHA is carefully controlled at low levels sufficient to overcome the iron deficiency without promoting a manganese deficiency. This characteristic of EDDHA is discussed in "Comm. in Soil Science and Plant Analysis", 4(1), 51-56 (1973).
It is accordingly an object of the present invention to provide a chelating agent which is useful in supplying nutrient metals to plants growing in calcareous soils. Another object of this invention is to provide metal chelants which are stable to hydrolysis in alkaline soils. It is a further object of this invention to provide a chelating agent which will supply adequate amounts of iron to chlorotic plants without creating a deficiency of other essential metals. These and other objects of this invention will be apparent from the ensuing description.